ON F, ECHINUS HYBRIDS. 465 



reared, an examination of the characters of the fully grown urchins 

 should decide whether the intermediate forms between the two species, 

 which are found in the sea and which are quite fertile, are to be con- 

 sidered as hybrids or as extreme variants of one of the two species. 



Besides making the cultures described above, I fertihzed E. 7niliaris 

 eggs with the sperm of the EA male, and used E. miliaris sperm to 

 fertilize EA eggs. This was done in order to see whether the inheritance 

 of the late larval characters (posterior epaulettes and green pigment) 

 in these crosses would be the same as when pure E. esculentus or E. acutus 

 was crossed with E. iniliaris. Now, twenty-one cultures,* derived from 

 fifteen fertilizations, have shown that the inheritance of these larval 

 characters has this year been the same as it was in 1912 : the E. escu- 

 lentus or E. acutus characters are developed in the hybrids in both 

 reciprocal crosses with E. miliaris. It was found that the two reciprocal 

 combinations of EA X miliaris likewise gave this result. From the cross 

 EA ? X miliaris <? large numbers of vigorous fully formed plutei de- 

 veloped, and a number of these " triple- hybrids " have already passed 

 through metamorphosis. 



Unfortunately the Fg generation obtained from the E. esculentus X 

 E. acutus hybrids can give no information as to the inheritance of the 

 late larval characters, since the latter are alike in the two species. It is 

 the F.j generation from hybrids between E. esculentus or E. acutus and 

 E. miliaris that will give this valuable information, but none of these 

 hybrids have as yet reached maturity. A small number of E. miliaris ? 

 X E. acutus c? hybrids (of which the largest measured 2| cm. in diameter, 

 exclusive of spines), from fertilizations made in May, 1912, were alive and 

 healthy this summer. After having tried unsuccessfully to induce these 

 to deposit eggs or sperm, I cut them open on June 6th of this year. They 

 contained, however, only small and quite immature gonads. 



As it must be some time before more E. acutus (or E. esculentus) X 

 E. miliaris hybrids will have grown large enough to be mature, I wish 

 to record these results up to date. The success in bringing the EA 

 hybrids to maturity has been largely due to the care taken by Mr. A. J. 

 Smith, head assistant at the Plymouth Laboratory, in attending to the 

 cultures after metamorphosis. The investigation was made with the 

 assistance of a grant from the Royal Society. 



* Some of these cultures were reared at Plymouth, others were transported as blastulai 

 to the Imperial College, London, and raised there in water which came from Lowestoft. 



NEW SEHIES. — VOL. X. NO. 3. OCTOBER, 1914. 2 G 



